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LMF 3

Former Member

Hi, newbie here.

I have a FARNELL LMF 3 audio sine wave geni', when using it last the transistors fitted in the AMPLIFIER board started to cook, I have the manual for it, changed the semiconductors and most of the components around the transistors, checking their values and most of them were within tolerance band.

Went overboard and changed all of the electrolytic caps including the smoothers in the psu.

Still getting the cooking smell, should have said at the beginning, all the voltages are spot on.

don't have enough hair to pull out , so can anybody point me in the right direction please.

regards, snowy (g0hze)

  • Hi Roland,

    Can you post pictures, schematic, etc. and identify which parts are getting hot. I used to have the same problem in the past when I was servicing audio equipment. I would get an amp with blown outputs. I would replace the outputs and related parts, plug it in, turn it on, and watch as my investment in parts went up in smoke. Later I would learn that even if the voltages appeared close it doesn't take much to bias a transistor too far into the saturation region and thus create current that are too high to sustain. Looking forward to looking more closely at your problem.

    John

  • in reply to jw0752

    Hi John, 1st of all let me correct the model number , it is a FARNELL LFM3, sorry for the grey cell malfunction I have ( hopefully) attached some pictures of the unit and the daughter boards, I have replaced all the semi conductors and the capacitors, also some discoloured resistors for exactly the right value components, spent a fair bit of folding stuff trying to correct the transistor overheating problem.

    Note to moderator, I apologise if I have transgressed by adding too big, too many pictures

    cheers

    Snowy@

    imageimageimageimageimage

  • in reply to Former Member

    Hi Roland,

    Which components specifically are getting too hot? Is it affecting the operation of the unit? Is there a schematic in your manual? The pictures look good and are helpful. The site does not seem to mind lots of pictures so post away.

    John

  • in reply to Former Member

    I don't have experience of servicing equipmemt like this, so take more note of what JW says than me, but my first thought would be to wonder about the biassing for the output stage. Is it the two transistors with the largest heatsinks on the board marked 'AMPLIFIER' that are cooking?

     

    If you have substituted new parts, the quiescent current through the pair of output transistors would need to be set again, wouldn't it?